1. Field of the Invention
This invention relates to the field of telecommunications
More precisely, the invention proposes a device and a method of encoding and decoding suited especially to a wire high-speed data transmission, particularly in a home or office environment.
2. Description of the Prior Art
Certain constraints have to be met during the data-sending and data-transmission phase for data transmission applications in interior and exterior environments. These constraints particularly limit the radiation spectrum. They are applicable particularly to transmission in a home or office environment. Thus, according to the prevailing standards of electromagnetic compatibility, the radiation spectrum must remain within a jig that restricts the radiation to within 30 MHz. Certain encoding methods are used for data transmission in an intra-building environment, particularly in order to limit radiation to within 30 MHz, while maintaining a high bit rate or speed.
Thus a first encoding system, the MLT3 (MulTiLevel 3) encoding system more particularly described in the IEEE802.3 standard, is used for example in 100BaseT type interfaces. In this encoding system, bits equal to “1” correspond to a transition, while bits equal to “0” correspond to an absence of transition. Thus, only bits equal to “1” will cause a change in the state signal. They are encoded successively on three states such as, for example, −V, 0, +V. Bits equal to “0” are encoded, for their part, in retaining the previously transmitted value.
The main advantage of the MLT3 encoding system is that it greatly reduces the frequency necessary for a given speed or bit rate, due to the use of three states. For example, for a bit rate of about 100 Mbits/s, the maximum parasite frequency of the signal is about 25 MHz.
One major disadvantage of this encoding system is caused by the long sequences of “0” bits that appear in this code and that can cause a loss or a phase shift of the receiver clock. To overcome this problem, encoded pieces of data are scrambled before being transmitted. This diminishes the improvements given by this code.
Moreover, since the Hamming distance between valid code words is very small (equal to one), transmission errors are facilitated, as errors might occur in encoding or in scrambling. There is then a risk of accepting invalid code words or rejecting valid code words.
Finally, this code makes it difficult to control the radiation spectrum and does not have particularly worthwhile error detection or error correction properties.
The prior art also envisages other encoding methods to obtain a radiation spectrum that partly meet the requirement of limiting radiation to within 30 MHz.
These encoding methods can be used to cancel the DC component of data to be transmitted, taken as an average or for each symbol transmitted.
The cancellation of this DC component (also known as DC balancing or direct-component balancing), provides a means of obtaining a radiation spectrum partly complying with a jig laid down by intra-building transmission standards, limiting radiation to within 30 MHz.
For example, one such method uses the Widmer and Franaszek encoding (A. X. Widmer and P. A Franaszek, IBM Journal of Research and Development, Vol. 27 No. 5, p. 440, September 1983), used in the IEEE 1394b standard. This encoding gives 10-bit words from 8-bit words, using a DC balancing technique.
This technique cancels the average DC component, and not the DC component for each transmitted symbol.
However, it has the disadvantage of permitting successive transitions. The result is non-negligible radiation beyond 30 MHz. The spectrum then does not fully comply with the jig laid down in frequency emission standards. Therefore this code only partially meets the specified constraints.